Patent GB-931926-A (GB)
Title: Improvements in turbine engine fuel control
IPC: F02C9/32
Filed: 1961-10-20
Abstract: 931,926. Gas-turbine engine fuel systems. GENERAL ELECTRIC CO. Oct. 20, 1961 [Oct. 26, 1960], No. 37708/61. Class 110 (3). [Also in Group XXIX] A gas-turbine engine fuel control system comprises a metering valve 29 to control the fuel supply at flow rates variable in accordance with the position of a throttle lever 27, there being a three-dimentional cam member 119 having at least two cam surfaces which is positioned axially as a function of engine speed and rotationally as a function of another parameter of engine operation for positioning at least two cam followers 125, 127 and associated levers to control the metering valve. The invention is characterized in that a spring- biased floating lever 153 is provided and abuts an adjustable pivot 157 intermediate its ends and is connected at one end through a force ratio varying means which provides compensation for the ambient pressure and compressor discharge pressure differential to the metering valve, the floating lever being connected at its other end to a rod 149, this end of the lever being axially movable thereby, the rod being provided with two or more abutments 147, 165, 171 in accordance with the desired control by parameters of engine operation for engagement by at least one of the positioned levers thereby determining the axial position of the rod for controlling the fuel flow through the metering valve. The engine is indicated at 11 and fuel is supplied to the combustion chamber by means of a pump 25 which supplies fuel through the metering valve 29 and line 21 to the burners. The metering valve comprises a piston valve 31 provided with an orifice 43, the space at the left-hand side of the piston being supplied with fuel from the pump 25 through duct 37. Fuel passes through the orifice 43 to the space at the right-hand side of the piston and leaks from there through orifice 45 the area of which is controlled by a pivoted bell-crank lever 47. The pressure drop across the metering valve 29 is maintained constant by means of a spring- loaded valve which controls flow of fuel through a by-pass passage 61 back to the pump inlet, the pressure of fuel upstream of the metering orifice being communicated to one side of the valve 57 through duet 59 and the pressure of fuel downstream of the orifice being communicated to the other side of the valve through orifice 63. The bell-crank lever 47 is loaded in an anti-clockwise direction by a spring 51 which is connected to the rod 53 of the metering valve 31, and in a clockwise direction by the rollers 71 of a force multiplication ratio varying means, the rollers bearing also against a pivoted lever 69 the position of which is controlled by the bellows assembly 67. The rollers 71 are mounted on a rod 73 which is movable axially by means of the floating lever 153, axial movement of the rod 73 to the left being limited by a stop 167, this stop constituting a deceleration fuel flow limit. The bellows assembly 67 comprises a pair of bellows 75, 77 disposed within a chamber which is open to ambient pressure through opening 81, the bellows 75 and 77 being interconnected by a rod which is connected to the opposite end of the pivoted lever 69. The bellows 74 is evacuated while the bellows 77 is connected through line 79 to compressor discharge pressure. The control system also comprises a speed-sensing device in the form of a centrifugal governor 89, the flyweights acting against spring 93 to control the position of a pilot valve 91 which controls supply of servo-fluid through ducts 97, 99 to one side or the other of a power piston 101 which controls the axial position of a rod 107, the rod 107 having pivoted thereto a feedback lever 111 which acts through rod 115 to control the loading of speeder spring 93. A threedimensional cam member 119 is secured on the rod 107, the cam member having a pair of cam surfaces 121, 123 which co-operate with followers 125, 127 respectively, the follower 125 being normally operative during steady-state operation and the follower 127 being operative during transient conditions. The follower 125 is carried by a lever 126 pivotally mounted at 128 on an axially-slidable rod 129, the lever 126 at its other end carrying a stop 145 which co-operates with an abutment 147 at one end of an axially-movable rod 149. The rod 129 is pivotally connected at one end to a speed reset lever 131 which in turn is pivotally mounted upon an adjustable pivot assembly 137, the opposite end of the lever 131 having a cam follower 133 which co-operates with a speed re-set or throttle cam 135 which is connected with the throttle lever 27. The rod 149 carries adjacent its other end a collar 151 in which engages the forked end of the floating lever 153, the other end of the lever having a pin and slot connection with the push-rod 73 carrying the roller assembly 71. The floating lever 153 engages against a fixed pivot 157 intermediate its end and is loaded against the pivot by means of a spring 159. The acceleration fuel limit is provided by the second cam surface 123 of the three-dimensional cam 119, the cam follower 127 which engages this surface being on a bell-crank lever 161 pivotally mounted at 163, the lever 161 engaging at its other end with a second abutment 165 carried by the axially-movable rod 149. The lever 161 therefore limits movement of the rod 149 towards the left which is the direction for increased fuel flow. as would be called for during engine acceleration. Deceleration fuel flow limiting is provided by the stop 167 which serves to limit movement of the roller assembly 71 to the left. A turbine temperature limit is provided by a second bell-crank lever 169 which may be moved into engagement with a third abutment 171 on the rod 149, the lever 169 being controlled by a cam 175, the position of which is dependent on the temperature of the gases downstream of the turbine rotor as sensed by a thermocouple 181. The threedimensional cam 119 is mounted for rotation on the rod 107 by means of a pin-and-slot connection 183, the cam being rotated by means of an actuator 185 which comprises a bleed servo, the bleed valve 189 of which is controlled by a lever 199 responsive to air temperature at the compressor inlet. The assembly comprises a temperature-responsive bulb 191 mounted in the compressor intake and connected by capillary tube 193 to the bellows 195, the bellows being pivotally connected to the lever 199. A second bellows 201 provides compensation for ambient temperature variation at the sensing bellows 195. The bellows 195 and 201 are loaded by a spring 203. The device acts to position the three-dimensional cam 119 as a direct function of the compressor inlet temperature.